The present invention relates to a method and apparatus for the continuous discharge of water from a suspension, especially from fibrous pulp, whereby pulp is thickened without the water being filtered through a thick, uncontrollably gathered fiber mat.
In the pulp and paper industry there are processes which are carried out with a low consistency of pulp, even under 1%. Such processes are, for example, normal and reverse vortex cleaning. Subsequent to the vortex cleaning the pulp is led to a processing stage, i.e. for example, to a thickener or a head box of a paper or drying machine In any case, thickening always follows the vortex cleaning in the process.
The treatment of fibrous material, especially cellulose and wood fiber material often takes place, as mentioned above, in a low consistency suspension. For example, screening with perforated or slotted screens is carried out with a consistency of 1 to 3%. Subsequent to the screening the fibrous material is thickened to a higher consistency for several reasons. Often the consistency is raised to the range of 10 to 15%, for example, for storage or bleaching.
Thickening is carried out according to modern techniques by means of different types of disc or drum thickeners and curved filters. In conventional drum and disc thickeners the discharge of liquid, in other words thickening, is based on so called "gravity deckers", vacuum filters or pressure filters.
With gravity deckers the thickening is carried out by means of a horizontally mounted drum made of perforated plate covered with wire cloth. The pressure difference required for the thickening results from the level difference between the pulp in the inlet tower and the pulp in the filtrate chamber. Pulp may be filtered either from the inside of the drum to the outside or from the outside to the inside, which latter direction is the most usual. In practice the diameter of the drum may be 4 m, of which, for example 60% is underwater. The maximum pressure difference is thus about 20 kPa. The pressure difference at the bottom dead center is zero, of which the difference increases to its maximum value towards the surface of the inlet tower. This results in that no thickening takes place on either side close to the bottom dead center. The situation is similar in the part of the drum which is not underwater. A considerable part of the drum surfaces of the gravity deckers is inefficiently utilized. The capacity of the part of the drum in efficient use also varies according to the pressure difference, which prevails relative to the filtering surface. The specific thickening capacity of gravity deckers varies according to the pulp and the running conditions, but is typically 400-700 1/M.sup.2 /min. Such types of thickeners are used to prethicken low consistency pulp, for example, from 0.5% to 1,5-5%.
The filtering surface of the drum is kept clean or open to the flow by moving the surface against the filtrate or by using air to clean it. For example, a mill producing 500 tons of 90% consistency pulp requires a filter with a diameter of 4 m and length of 7 m, the surface area being about 88 m.sup.2 of wire surface, to thicken the pulp from 0,5% to 1,5%.
The thickening method using a curved filter is based on gravity decker filtering. The suspension to be thickened is pumped onto an inclined filtering surface. The thickening capacity is in practice 3 to 5% and the specific capacity of liquid discharge is about the same as that of the drum filters. It has the advantage of not having any mechanically moving members, but it also has the drawback of the apparatus being very easy to clog, because arranging for efficient cleaning is difficult. Curved filter type thickeners are used in the pulp and paper industry, when minor thickening and low pulp capacities are concerned.
The above described conventional pulp thickening apparatuses or "thickeners" are characterized in that the thickening is carried out using very small pressure differences in more or less open equipment and only part of the filtering surface is utilized.
The small pressure difference and the partial use of the filtering surface result in a poor ability to discharge liquid. The open construction and operational principle result in the pulp and the filtrate possibly including air. Air in the pulp weakens, as is known, the infiltration qualities of the pulp decisively.
Of other arrangements applied in the prior art, different types of vacuum filters are used most frequently. The consistency of pulp in these filters is caused to increase by removing water from the pulp through a filter surface, for example, through a wire cloth covered by a thick fiber mat In thickening pulp it is possible, by means of the suction effect on the pulp, to use a maximum pressure difference of about 0,5 bar, because a stronger vacuum would make the filtrate boil, which is undesirable.
The pressure difference required for filtering in vacuum thickeners and disc thickeners is achieved by a suction leg. Such a thickener differs from the gravity deckers in that a pulp layer is formed for them. This means that subsequent to thickening the consistency of pulp is 8 to 14%. The capacity of a vacuum or disc thickener is about the same as that of a gravity decker The difference is that the pulp web is formed by suction on the filtering surface by the pulp suspension when said surface is underwater. From the web formed on the part of the drum when such has risen above the surface of the suspension, filtrate is removed so as to achieve the consistency of said 8 to 14% in the discharge. It is clear that when forming a fiber mat on the filtering surface, the discharge of liquid through the layer substantially slows down due to the great flow resistance of the filter web.
It is not advantageous to use this type of thickener when tending to prethicken, but they may be applied when the discharge consistency required is high. The specific thickening capacity varies according to the quality of pulp, and conditions 50-300 l/m.sup.2 /min. Compared to the above example two vacuum filters of said size would be required when aiming for a consistency of 10%. The advantage of a disc filter compared with a vacuum drum filter is that more filtering surface can be included in the same volume.
A pressure filter differs from the vacuum drum filter in that the filtering pressure difference is generated by pressure.
The problem with these and many other types of thickeners is their tendency to clog. As an example, a situation can be mentioned, in which the pressurized suspension to be thickened is led to a thickener, whereby, in principle, the pressure difference is unlimited. In laboratory tests this type of thickener was clogged by sulfate pulp in ten seconds, after which it had to be cleaned.
Several methods are known to prevent the clogging or to loosen the web from the filter. For example, in FI patent specification No. 41712 and U.S. Pat. specification No. 3,455,821 the purpose is to clean the filter surface by vibration. However, the damping ability of the gaseous and fibrous paper pulp prevents the cleaning effect of the vibrations.
One method is shown in FI specification No. 68005, according to which cleaning of the disc filter is carried out by using compressed air. At a certain stage of the disc sector circulation, compressed air is led to the inside of the disc sector, whereby the blast loosens the filtered pulp from outside the sector.